[Agda] Agda documentation

[Peter Selinger]

I completely agree! Documentation is key. Thanks for putting effort
into it! -- Peter

Wouter Swierstra wrote:
> 
> > In case you're not following the github repository closely, you might not
> > know about the documentation project that is slowly taking shape at
> > http://agda.readthedocs.org. I just added a section on with-abstraction
> > that might be of interest to people:
> >
> 
> Thanks for doing this Ulf (and others)! The documentation has been lagging
> behind Agda's implementation for a while now. Having more structured
> documentation going beyond the release notes is super valuable.
> 
>   Wouter